This invention relates generally to firing circuits for power thyristors, and more particularly, to a firing circuit wherein the gate of the firing thyristor is connected to the first terminal of at least one auxiliary thyristor, the breakover voltage of which is smaller than the breakover voltage of the power thyristor, and which is fired by normal firing pulses.
A firing circuit which is provided with an auxiliary thyristor which is connected to the gate terminal of a power thyristor is described in U.S. Pat. Nos. 3,836,994 and 3,662,250. In the known arrangement, the anode-cathode path of the power thyristor is shunted by a series circuit consisting of an overvoltage detecting device and an energy storage device. The junction points of these two devices is connected to the gate of the power thyristor. If an excessively high voltage is applied to the power thyristor, the overvoltage detecting device becomes conductive and provides a trigger singal to the gate of the power thyristor. The power thyristor is fired before the applied voltage can reach a critical value. The gate current which is supplied by the overvoltage detecting device rises initially very steeply, because the energy storage device has an inductance which prevents it from drawing current rapidly. A portion of the current which is supplied by the overvoltage protecting device is conducted to the energy storage device only after a predetermined amount of time delay. If the voltage at the power thyristor breaks down, the energy storage device serves as a current source for addressing the power thyristor, and thereby maintains a trigger signal of sufficient amplitude and duration to assure the switching-on of the power thyristor. The overvoltage detecting device can be constructed with one or more series-connected auxiliary thyristors. The firing pulses for normal firing can then be conducted to the gates of the auxiliary thyristors, such that normal firing is accomplished via the auxiliary thyristors.
The known circuit is suitable for servicing several power thyristors which are connected in parallel with the same direction of conduction. If, however, the thyristor device is to be operated in two current directions, the power thyristors must be connected antiparallel. In this case, the known firing circuit becomes relatively complicated since the entire firing circuit, with the overvoltage detecting device and the energy storage device, must be provided in duplicate.
A firing circuit for a power thyristor having a gate which is connected to a first terminal of a threshold element for the protective firing of the power thyristor in the event of an overvoltage is described in the journal "Siemens-Energietechnik 2" 1980, no. 10, page 420. A second terminal of the threshold element having threshold voltage characteristic which is smaller than the zero breakover voltage of the power thyristor receives the anode voltage of the power thyristor. The firing command for the power thyristor is transmitted in the form of light pulses via high-voltage-proof light guides from ground potential to the thyristor potential; which firing command is converted via a receiving circuit into electric driving pulses for the power thyristor. The energy required for driving the thyristor is obtained from the anode-cathode voltage thereof. In order to protect the power thyristor in the event that the zero breakover voltage is exceeded, a breakover diode is connected between the anode and the gate of the power thyristor. If a voltage threshold is exceeded, this diode becomes conductive and delivers, regardless of a normal drive of the power thyristor, a driving pulse which switches the power thyristor into conduction and thereby allows the voltage present to be reduced. In this manner, a highly effective protection of the power thyristor is obtained for positive overvoltages. In addition, the reliability of the thyristor valve is increased if several power thyristors are connected in series in a thyristor valve. This results from the fact that even if the driver circuitry is defective the corresponding power thyristor continues to participate in the operation of the thyristor valve without undue overstress because it is fired at the same time with the firing of the other power thyristors of the thyristor valve by the overvoltage. In this known circuit arrangement, therefore, separate components are required for normal firing and for protective firing in the event of overvoltage.
It is, therefore, an object of this invention to provide a firing circuit of the type discussed hereinabove wherein the firing of anti-parallel connected power thyristors is achieved without undue expense and circuit complexity.